The present invention relates to drilling apparatus for use in exploring underground petroleum reserves and more specifically to a novel improved means for mounting the rotary cutters on the trunnion of the rotary drill bit.
In the prior art, it is known to mount the rotary cutters on the trunnion by means of a slip ring of lubricant-holding material, such as a self-lubricating plastic, which engages one surface of the rotary cutter and the trunnion of the rotary drill bit in a circular groove. This arrangement is shown in German Pat. No. DE-PS 1 141 962. A disadvantage of this known design is that in order to accommodate the slip ring, it is necessary to form a relatively deep circular groove in the slip surface of the trunnion which tends to weaken the trunnion. A further disadvantage is that the slip ring displaces part of the radially supporting slip surface of the friction bearing so that the bearing has a comparatively small load carrying capacity. Additionally, in this prior known arrangement, there is a very little space to accommodate storage of lubricant in the bearing cavity of the rotary cutters so that the surface life of the rotary drill bit is limited by reason of bearing failure due to poor lubrication. It has been found that the problem cannot be alleviated by simply increasing the size of the parts to increase the strength of the rotary drill bit and enlarge the lubricant storage cavity since the size of the entire assembly is limited by the diameter of the bore hole which must be kept small for economic reasons.
In veiw of the above, an object of the present invention is to provide a compact rotary drill bit with plural rotary cutters which have a high load carrying capacity and high strength.
It is also an object of the present invention to provide an assembly which ensures adequate lubrication of the bearing over a prolonged period of time and incorporates a comparatively simple and effective means for securing the rotary cutter against axial displacement from the trunnion.